To release:

Change the version code in the app/build.gradle file; commit and push.
Build signed APK.
In Github Releases, draft a new release with tag v(major).(minor).(patch) to match what you did above.
Write your changelog in the description. Markdown is supported.
Upload APK to github releases.
Release.
Delete the last release's APK, to prevent previous version support as much as posible.